Alumina-zirconia-silicon carbide-magnesia ceramic cutting tools
This is a continuation of copending application Ser. No. 07/266,959 filed on Nov. 3, 1988 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to alumina based ceramic cutting tools containing zirconia and silicon carbide. It especially relates to ceramic cutting tools useful in the high speed, rough machining of ferrous and nonferrous metals and alloys. 
     In the past, zirconia has been added in specified amounts to selected alumina-silicon carbide whisker reinforced compositions described in U.S. Pat. Nos. 4,534,345 and 4,507,224 to provide enhanced fracture toughness and/or flexural strength (see: U.S. Pat. Nos. 4,657,877 and 4,749,667; Japanese Patent Publication No. Sho 62-265182; Clausen et al, &#34;Whisker-Reinforced Oxide Ceramics,&#34; Journal de Physique Colloque Cl, Supplement au No. 2, Tome 47, February 1986, Pages C1-693 to C1-702; Becher et al, &#34;Toughening of Ceramics by Whisker Reinforcement,&#34; Fracture Mechanics of Ceramics 7, ed. by Bradt et al, Plenum Press, New York (1986), Pages 61-73). 
     It has been indicated that the zirconia should be in the monoclinic and/or (metastable) tetragonal phase to obtain improved fracture toughness and/or flexural strength. It has been further indicated that the metastable tetragonal phase is obtained by reducing zirconia particle size or through the use of a cubic zirconia stabilization promoter such as yttria, calcia, magnesia and the rare earth oxides in amounts below that required to fully stabilize the cubic zirconia phase. 
     Cutting tools composed of a variety of compositions containing alumina, zirconia and silicon carbide whiskers with or without other additives have been proposed (see: European Patent Application No. 86107916.8 (published Jan, 21, 1987 as No. 0208910); U.S. Pat. No. 4,749,667; &#34;Multitoughening Ceramic,&#34; Techno Japan, Vol. 19, No. 10, October 1986, Page 78; and European Patent Application No. 86301597.0 published Sept. 17, 1986, as No. 0194811). 
     None of the foregoing documents teaches or suggests that, for metalcutting inserts for use in the high speed roughing of ferrous and nonferrous metals and alloys, cutting performance can be significantly improved by controlling the alumina based ceramic composition to within the combination of critical ranges now discovered by the applicants for zirconia, magnesia, silicon carbide and tetragonal zirconia contents. 
     SUMMARY OF THE INVENTION 
     The present invention provides alumina based ceramic cutting inserts containing (in volume percent, v/o) about 1.5 to 37.5 v/o (volume percent) silicon carbide whiskers, about 5 to 17.5 v/o zirconia, the residue of a magnesium oxide or other magnesium-oxygen compound addition, and at least 2 v/o tetragonal zirconia. It has been surprisingly found that, despite the finding that minor magnesia additions act to reduce the amount of tetragonal (i.e., metastable tetragonal) zirconia at room temperature, an effective amount of this addition has a significant positive affect on cutting edge lifetime in the high speed rough turning of soft steels, such as AISI (American Iron and Steel Institute) 1045 steel. 
     It is preferred that magnesia be added in amounts of about 0.03 to 3 v/o; more preferably, about 0.03 to 2.0 v/o and most preferably, about 0.04 to 1.0 v/o. 
     Preferably, the alumina based ceramic composition according to the present invention contains about 2.5 to 35 v/o and, more preferably, about 5 to 32.5 v/o silicon carbide whiskers. 
     The zirconia content according to the present invention is preferably 7.5 to 17.5 v/o and, more preferably, 10 to 15 v/o. In accordance with the present invention, a significant fraction of the zirconia is in the form of tetragonal zirconia and must form at least 2 v/o of the composition. Preferably, at least 4 v/o; more preferably, at least 6 v/o; and most preferably, at least 8 v/o of the composition is in the form of tetragonal zirconia. 
     These and other aspects of the present invention will become more apparent upon review of the detailed description of the present invention in conjunction with the figures briefly described below: 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of an embodiment of a square cutting insert in accordance with the present invention. 
     FIG. 2 shows a graph of the volume percent tetragonal zirconia in the composite as a function of the volume percent of magnesia, or yttria, addition. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the present invention, as shown in FIG. 1, metalcutting insert 10, preferably of the indexable type, is provided having a rake face 30, a flank face 50 and cutting edge 70 at the juncture of the rake and the flank faces. The cutting edge 70 is preferably in a chamfered condition (e.g., K-land) as shown in FIG. 1. The metalcutting insert is composed of an alumina based ceramic composition containing: about 1.5 to 37.5 v/o silicon carbide whiskers; about 5 to 17.5 v/o zirconia; and a residue of a magnesium oxide or other magnesium-oxygen compound addition added in an amount effective to enhance the metalcutting lifetime of the cutting edge. The silicon carbide whiskers, zirconia and residue of magnesia are substantially homogeneously distributed in an alumina based matrix. 
     The silicon carbide whiskers are present at a level of at least about 1.5 v/o to assure minimal levels of cutting edge lifetime improvement. More preferably, silicon carbide whiskers are present at about 2.5 v/o or more and, most preferably, at about 5 v/o or more. The silicon carbide whisker content preferably should not exceed about 37.5 v/o of the composition. We believe that silicon carbide whisker contents above this value result in a significant decrease in the cutting edge lifetime. Therefore, to further maximize cutting edge lifetime during high speed rough turning, it is preferred that the maximum content of silicon carbide whiskers be held at or below about 35 v/o and, more preferably, 32.5 v/o of the alumina based ceramic composition. The optimum silicon carbide whisker level will depend upon the application. 
     The silicon carbide whiskers utilized herein may, for example, be any of the commercially available brands which have been used in the past in alumina based metalcutting inserts for machining nickel base superalloys. 
     While less preferred, silicon carbide particles of a generally equiaxed shape or platelet shape may be substituted for part of the silicon carbide whiskers in this invention. 
     The zirconia content is in the range of about 5 to 17.5 v/o of the ceramic composition. Zirconia contents outside of this range are believed to provide compositions having reduced cutting edge lifetime during the high speed roughing of AISI 1045 steel. Preferably, to maximize cutting edge lifetime, the zirconia content should be within the range of about 7.5 to 17.5 v/o and, more preferably, about 10 to 15 v/o of the composition. While we believe that the concentration of tetragonal zirconia should be maximized for best cutting performance, it is equally, if not more, important in our opinion that as much of the tetragonal zirconia as possible, present at room temperature, be available for transformation toughening at, or as near as possible to, the temperatures encountered at the cutting edge during machining. It is, therefore, critical to the present invention that magnesia be present in certain small, but effective, amounts which have been found critical to the maximization of cutting edge lifetime. In accordance with the present invention, at least 2 v/o of the ceramic composition, must be tetragonal zirconia. Preferably, the tetragonal zirconia forms at least about 4 v/o of the composition, more preferably, at least about 6 v/o of the composition, and most preferably, at least about 8.0 v/o of the composition. 
     Magnesia additions, despite the fact that they decrease the amount of tetragonal zirconia observed at room temperature, are added in the range of about 0.03 to 3 v/o of the composition, preferably, about 0.03 to 2 v/o; more preferably, about 0.04 to 1.0 v/o. Magnesia may be blended in with the alumina or zirconia just prior to compact pressing, or it may be preblended or prealloyed with the alumina or zirconia. Preblending of the magnesia powder is preferred since it is believed that the preblended magnesia is more effective in producing high temperature metastable tetragonal zirconia, thus allowing a smaller amount of magnesia to be added and minimizing the deleterious effects of high magnesia additions (e.g., lower melting point glass and Mg-Al-O formations). Equivalent amounts of other magnesium-oxygen compounds, such as magnesium carbonate, which may require an additional processing step such as calcination to produce magnesia, may be substituted for all or part of the magnesia addition. After sintering of the blended compositions, the magnesia addition may not exist as a separate phase but as a residue. This residue may include, for example, magnesium aluminate, magnesia alumina solid solution, a magnesia zirconia solid solution and/or a glass, for example, formed with silicon dioxide impurities which may have existed as a thin coating on the silicon carbide whiskers. 
     The remainder of the ceramic composition is essentially alumina and preferably entirely alumina except for impurities. In all cases, the present alumina based ceramic composition contains at least 40 v/o alumina, and preferably at least 50 v/o alumina. 
     Titanium carbide, as whiskers and/or substantially equiaxed particulate, may be added in an amount of about 2-35 v/o of the composition and, preferably, about 10 to 30 v/o. Titanium carbide has a higher thermal expansion coefficient than alumina. It is, therefore, believed that titanium carbide additions should allow more tetragonal zirconia to be retained at room temperature. Titanium carbide whiskers may be manufactured and harvested by the methods described in A. Kato et al, &#34;Growth Rate of Titanium Carbide Whiskers in Chemical Vapor Deposition,&#34; J. Cryst. Growth, 37 (1977), Pages 293-300; and N. Tamari et al, &#34;Catalytic Effects of Various Metals and Refractory Oxides on the Growth of TiC Whiskers by Chemical Vapor Deposition,&#34; J. Cryst. Growth, 46 (1979), Pages 221-237. Titanium carbide whiskers and their incorporation and use in alumina based cutting inserts are disclosed in Mehrotra et al U.S. patent application Ser. No. 056,091, filed May 28, 1987, and assigned to Kennametal Inc., now U.S. Pat. No. 4,852,999. 
     The alumina powders utilized herein should be high purity alumina (i.e., &gt;99% pure) such as produced by ALCOA (e.g., grade A16SG), or by Ceralox (e.g., grade HPA - 0.5 with or without magnesia), or by Reynolds Chemicals (grade RC-HP or RC-HP-DBM). 
     Yttria, calcia, the rare earth oxides, and other compounds which have, through a reduction in the tetragonal to monoclinic transformation temperature, an adverse affect on cutting edge lifetime are preferably present only as impurities, if present at all. 
     The foregoing material, in accordance with the present invention, may be milled, blended, and densified at high temperature to produce at least 98%, and preferably, at least 99% dense alumina based ceramic compositions having an alumina based matrix, which is preferably entirely alumina, in which the silicon carbide, magnesia residue from the magnesia addition, zirconia and titanium carbide, if any, are at least substantially homogeneously distributed. Hot pressing temperatures are preferably held below 1750° C., and more preferably, below 1650° C. and, most preferably, below about 1600° C. to minimize zirconia particle growth and thereby maximize the tetragonal (i.e., metastable tetragonal) zirconia phase present in the final product. The average zirconia particle size in the cutting insert should not exceed about 5 microns, preferably should not exceed 2 microns, and more preferably, should not exceed 1 micron. However, the average zirconia particle should be large enough to allow most tetragonal zirconia to transform to monoclinic zirconia during use. This minimum size will depend upon the ceramic composition and is presently undetermined. 
     While not wishing to be bound by any particular theory, applicants offer the following explanation of the present invention. In alumina-silicon carbide whisker-zirconia compositions, the amount of metastable tetragonal zirconia that can be obtained at room temperature can be increased by a reduction in zirconia particle size or the addition of the so-called cubic stabilizing agents, such as yttria, calcia and/or the rare earth oxides. (See: Stevens, &#34;An Introduction to Zirconia--Zirconia and Zirconia Ceramics,&#34; Magnesium Elektron Pub. No. 113, Magnesium Elektron Ltd., England (1986)). While the literature commonly includes magnesia among the foregoing list of stabilizing agents, applicants have found that, when magnesia is added in the amount of about 0.03 to 3 v/o to the present compositions, magnesia decreases the amount of tetragonal zirconia present at room temperature. When yttria is added to zirconia, it tends to stabilize the tetragonal and cubic phases of zirconia to a lower temperature. All the aforementioned stabilizing agents, and most impurities, except for magnesia, affect ZrO 2  in similar ways (i.e., they reduce the temperature at which the tetragonal zirconia phase is stable). At room temperature, some of the zirconia may be present as metastable tetragonal zirconia. Under the action of tensile stress, this tetragonal zirconia may become monoclinic, giving rise to transformation toughening. However, as the temperature increases, tetragonal zirconia becomes stable and, therefore, unavailable for transformation to the monoclinic crystal structure. Thus, any impurity or additive, such as yttria, which stabilizes tetragonal zirconia at lower temperatures, is unsuitable for metalcutting applications since the cutting tip temperature may rise to about 1000° to 1200° C. in high speed machining. Therefore, in accordance with the theory of the present invention, additives which raise the monoclinic to tetragonal transformation temperature of the zirconia are required for high temperature transformation toughening. We have found that there are only two oxide additives, MgO and HfO 2 , which raise the transformation temperature. Thus, we believe that by keeping the zirconia particle size small, a large proportion of the zirconia can be retained as metastable tetragonal zirconia at the high temperature of metalcutting by alloying the zirconia with magnesia or hafnia. It is our belief that this helps obtain enhanced cutting edge lifetimes during metalcutting operations. It should be noted that zirconia normally contains up to about 2 w/o (weight percent) hafnia as an impurity. 
     The significant positive impact that magnesia additions have on metalcutting performance is more clearly indicated by the following examples which are purely illustrative of the present invention. 
     Six compositions were prepared (Table 1) with the nominal compositions being Al 2  O 3  - 10 v/o SiC w  (silicon carbide whiskers) - 10 v/o ZrO 2 . Small additions of Y 2  O 3  and MgO were made. In the case of Mix No. 6, about 0.05 w/o (approximately 0.06 v/o) MgO had already been blended with Al 2  O 3  by the powder manufacturer. This provided Mix No. 6 with a magnesia content of about 0.04 v/o. Fifty gram batches of these powders were prepared by first blending Al 2  O 3  and ZrO 2  (and stabilizing additives, if any) slurries (propanol) in a jar mill using Al 2  O 3  cycloids for one hour. Sonicated SiC w  slurry was then added, and the whole mix was blended for one hour. Al 2  O 3  and ZrO 2  slurries had previously been milled to obtain mean particle sizes of 0.5 to 0.6 μm and 0.6 to 0.8 μm, respectively (corresponding specific surface areas measured by BET were 10 to 14 m 2  /g and 20 to 40 m 2  /g, respectively). Then, the mix was pan dried, screened through 100 mesh screen, and hot pressed in a one inch diameter graphite die at the temperatures and pressures shown below in Table 1 for one hour in argon. The resulting billets were more than 99% dense, and were cut, ground and polished for measurement of physical and mechanical properties. The billets were also cut and ground to produce indexable cutting inserts for metalcutting tests. 
     
                       TABLE 1                                                     
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COMPOSITIONS                                                              
______________________________________                                    
Nominal Composition:                                                      
               Al.sub.2 O.sub.3 - 10 v/o SiC.sub.w - 10 v/o ZrO.sub.2     
Al.sub.2 O.sub.3 :                                                        
               Alcoa A16SG                                                
SiC.sub.w :    Tokai Carbon Co. (Tokyo, Japan)                            
               TOKAWHISKER (TOKAMAX)                                      
               Grade 1                                                    
               (0.3-1.0 μm diameter; 20-50                             
               μm length)                                              
ZrO.sub.2 :    Zircar - unstabilized                                      
MgO:           Fisher Scientific Corp. - Reagent                          
               Grade BET = 40.4 m.sup.2 /g                                
Y.sub.2 O.sub.3 :                                                         
               MolyCorp, BET = 15.4 m.sup.2 /g                            
______________________________________                                    
Mix   Hot pressing                                                        
No.   Temp. (°C.)                                                  
                 Pressure (psi)                                           
                            Composition                                   
______________________________________                                    
1.    1650       4000       Nominal                                       
2.    1650       4500       Nominal + 1 v/o Y.sub.2 O.sub.3               
3.    1625       4500       Nominal + 1 v/o MgO                           
4.    1600       4500       Nominal + 1 v/o Y.sub.2 O.sub.3 +             
                            1 v/o MgO                                     
5.    1625       4500       Nominal, except that                          
                            Al.sub.2 O.sub.3 used was Ceralox             
                            Grade HPA - 0.5 (0.5                          
                            to 0.7 μm median                           
                            particle size)                                
6.    1625       4500       Nominal, except that                          
                            Al.sub.2 O.sub.3 used was Ceralox             
                            Grade HPA - 0.5 with                          
                            MgO (0.05 w/o)                                
______________________________________                                    
 
    
     
                       TABLE 2                                                     
______________________________________                                    
PROPERTIES                                                                
                    Tetragonal ZrO.sub.2.sup.2                            
Mix   RA       Fracture Toughness                                         
                              as v/o as v/o of                            
No.   Hardness K.sub.IC (E&amp;C).sup.1 (MPa m.sup.1/2)                       
                              of ZrO.sub.2                                
                                     Composite                            
______________________________________                                    
1     93.3     5.59           84     8.4                                  
2     93.3     4.97           100    10                                   
3     93.6     5.96           70     7                                    
4     93.6     4.63           98     9.8                                  
5     93.9     5.88           84     8.4                                  
6     93.8     5.88           76     7.6                                  
______________________________________                                    
 .sup.1 Evans and Charles, &#34;Fracture Toughness Determination by           
 Indentation,&#34; J. American Ceramic Society, Vol. 59, No. 7-8, Pages 371,  
 372, using an 18.5 kg load.                                              
 .sup.2 As measured by xray diffraction of a polished surface. The        
 remainder of the zirconia is assumed to be monoclinic zirconia. Cubic    
 zirconia, which may be present in minor amounts, is included within the  
 tetragonal zirconia estimate.                                            
 
    
     Physical and mechanical properties of the hot pressed composites are shown in Table 2. 
     The Porter-Heuer (Porter et al, J. American Ceramic Society, Vol. 62, No. 5-6 (1979), pages 298-305) formula was modified and used to estimate the fraction of monoclinic ZrO 2  (Vm) from peak intensities of the 111 reflection of the monoclinic ZrO 2  (Im(111)), and 111 reflection of the-tetragonal ZrO 2  (It(111)): ##EQU1## where Vt is the fraction of ZrO 2  which is tetragonal ZrO 2 . 
     The estimated volume fraction of tetragonal ZrO 2  in the whole composite is (vt): 
     
         vt=Vt vz                                                   (3) 
    
     where vz is the volume fraction of the total ZrO 2  added in the mix. The above relationship assumes that ZrO 2  substantially remains unchanged during consolidation except for the phase transformations discussed above. 
     FIG. 2 shows the effect of the various additives on the amount of tetragonal zirconia in the composites. It can clearly be seen that magnesia additions lower the amount of tetragonal zirconia (curve 1), whereas, yttria additions increase the amount of tetragonal zirconia at room temperature (curve2). 
     
                       TABLE 3                                                     
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TURNING AISI 1045* STEEL (192-200 BHN)                                    
______________________________________                                    
Insert    Cutting Edge Life &amp; Average Life                                
Material  Failure Mode        (minutes)                                   
______________________________________                                    
Mix 1     14. BK     14.7 BK  14.4                                        
Mix 2     8. BK      12.6 BK  10.3                                        
Mix 3     15.9 BK    29.2 FW  22.6                                        
Mix 4     1.7 FW     7.7 FW   4.7                                         
Mix 5     17. FW     7.5 BK   12.3                                        
Mix 6     22.9 DN    32.9 FW  27.9                                        
______________________________________                                    
Test Conditions:                                                          
1000 sfm (surface feet/minute)                                            
0.025 ipr (inch/revolution)                                               
0.100 inch doc (depth of cut)                                             
SNGN-453T (American National Standard                                     
Designation in accordance with ANSI B212.4 -                              
1986) indexable cutting insert style                                      
(cutting edge preparation:                                                
0.008 inch × 20° K-land)                                     
15° lead angle (side cutting edge angle)                           
-5° side rake angle                                                
-5° back rake angle                                                
no coolant                                                                
Cutting Edge Life Criteria:                                               
FW - .015&#34; uniform flank wear                                             
MW - .030&#34; concentrated flank wear                                        
CR - .004&#34; crater wear                                                    
DN - .030&#34; depth of cut notch                                             
CH - .030&#34; concentrated wear or chipping                                  
BK - breakage                                                             
______________________________________                                    
 *AISI 1045 is equivalent to Unified Numbering System (UNS) Designation   
 G10450.                                                                  
 
    
     Indexable insert cutting edge lifetimes in the high speed roughing of a premachined AISI 1045 steel are shown in Table 3. It can be clearly seen that a significant improvement in cutting edge life is achieved by the addition of magnesia, whereas, a decrease in tool life occurs when yttria is added despite the high level of tetragonal zirconia present in the yttria containing compositions. 
     
                       TABLE 4                                                     
______________________________________                                    
COMPOSITIONS                                                              
______________________________________                                    
Nominal Composition:                                                      
               Al.sub.2 O.sub.3 - 5 v/o SiC.sub.w - 10 v/o ZrO.sub.2      
Al.sub.2 O.sub.3 :                                                        
               Ceralox-HPA-.5 without MgO                                 
SiC.sub.w :    Tokai Grade 1                                              
ZrO.sub.2 :    Magnesium Elektron (SC15) -                                
               unstabilized (.5-.6 μm particle                         
               size BET 5-8 m.sup.2 /g)                                   
______________________________________                                    
       Hot Pressing                                                       
No.    Temp. °C.                                                   
                 Pressure (psi)                                           
                            Composition                                   
______________________________________                                    
7.     1535° C.                                                    
                 5000 psi   Nominal + .05 v/o MgO                         
8.     1550° C.                                                    
                 5000 psi   Nominal + .25 v/o MgO                         
9.     1550° C.                                                    
                 5000 psi   Nominal + .50 v/o MgO                         
10.    1550° C.                                                    
                 5000 psi   Nominal + 1.0 v/o MgO                         
11.    1550° C.                                                    
                 5000 psi   Nominal + 3.0 v/o MgO                         
______________________________________                                    
 
    
     A second series of mixes, 7 to 11 shown in Table 4, were made to further demonstrate the effect that the level of the magnesia has on tetragonal zirconia content and cutting edge lifetime. All samples were processed and hot pressed essentially as described with respect to the samples produced from Mixes 1 to 6. 
     The physical and mechanical properties of the materials are reported in Table 5. It can clearly be seen that the tetragonal zirconia content again clearly decreases with increasing amounts of magnesia addition. This affect is also shown in FIG. 1, curve 3. It can be seen that Mixes 7 to 11 have a higher tetragonal zirconia content than that found in the materials represented by curve 1. The affect is believed to be due to the lower SiC w  content (5 v/o versus 10 v/o) used in the second group of mixes. Applicants have observed that generally, as SiC whisker content increases, the amount of tetragonal zirconia decreases for a given zirconia content and zirconia particle size, and everything else being held constant. 
     
                       TABLE 5                                                     
______________________________________                                    
PROPERTIES                                                                
                                     Tetragonal                           
                                     ZrO.sub.2 as                         
Mix            RA       Fracture Toughness                                
                                     v/o of                               
No.  V/o MgO   Hardness K.sub.IC (E&amp;C) (MPa m.sup.1/2)                    
                                     Composite                            
______________________________________                                    
7    0.05      93.5     5.57         8.5                                  
8    0.25      93.4     5.00         8.1                                  
9    0.50      93.3     5.06         8.0                                  
10   1.0       93.4     4.75         7.6                                  
11   3.0       93.5     4.98         7.3                                  
______________________________________                                    
 
    
     lndexable insert cutting edge lifetimes in the high speed roughing of AISI 1045 steel are shown in Table 6. 
     
                       TABLE 6                                                     
______________________________________                                    
TURNING AISI 1045 STEEL (197-199 BHN)                                     
Insert    Cutting Edge Life    Average Life                               
Material  &amp; Failure Mode       (Minutes)                                  
______________________________________                                    
Mix                                                                       
7         19 bk       5 ch/bk  12                                         
8         12.6 dn/ch  1 bk     6.8                                        
9         12.3 dn     14.1 dn  13.2                                       
10        9.7 bk      7.1 dn   8.4                                        
11        7.0 bk      6.8 dn   6.9                                        
______________________________________                                    
 
    
     The test procedures and conditions and cutting edge life criteria used to generate the data shown in Tables 5 and 6 were the same as that described for Tables 2 and 3. 
     In another example in accordance with the present invention, a composition containing Al 2  O 3  -2.5 v/o SiC w  -10v/o ZrO 2  -1.05 v/o MgO was made. A fifty gram batch of this composition was prepared by first blending an Al 2  O 3  (Ceralax Grade HPA-0.5 with MgO (0.05 w/o)), ZrO 2  (Magnesium Elektron SC15) and MgO (Fisher Reagent Grade) slurry (propanol) in a jar mill using Al 2  O 3  cycloids for one hour. A Sonicated SiCw (Tokai Grade 1) slurry was then added and the whole mix was blended for one hour. The Al 2  O 3  and ZrO 2  containing slurries had previously been milled to obtain mean particle size of about 0.5-0.7 μm and 0.5 to 0.6 μm, respectively. Then the mix was pan dried, screened through a 100 mesh screen and isostatically compacted at 30,000 psi at room temperature. Pieces were then cut from the resulting cold compacted blank and sintered at 1700 degrees Centigrade for one hour in one atmosphere argon followed by hot isostatic pressing at 1600 degrees Centigrade for one hour in 17,000 psi argon. The resulting samples were greater than 99 percent dense (i.e., fully dense). As described in the prior examples, samples were then prepared for physical and mechanical testing and ground into indexable cutting inserts. It was determined that the samples contained about 6.6 v/o tetragonal zirconia. It is estimated that material processed in this manner has a zirconia particle size of about 5 μm, or less. Cutting inserts of the style described in Table 3 were tested under the conditions used in Table 3. Cutting edge life times of 14.4 (DN failure) and 18.9 (FW &amp; CH failure) minutes were obtained. 
     It is believed that cutting edge lifetime may be extended or made more uniform if the cutting edge is honed and/or the insert surface is lapped or polished to remove surface material containing a higher percentage of monoclinic and a lower percentage of tetragonal zirconia than is characteristic of the bulk of the material (i.e., a polished surface). It is known that grinding stresses create a surface layer in which a portion of the metastable tetragonal zirconia has been transformed to monoclinic zirconia. It is preferred that at least those surface areas of the insert which will encounter high temperatures during use have the maximum amount of tetragonal zirconia available for high temperature transformation. 
     While the foregoing examples have shown the value of the present invention in cutting soft steels, it should be understood that cutting inserts in accordance with the present invention may be used for machining other materials by optimizing the composition to provide best results. For example, for high speed rough turning of nickel base super alloys, compositions containing about 25 to 37.5 v/o silicon carbide whiskers may be most appropriate. 
     All patents, patent applications and documents referred to herein are hereby incorporated by reference. 
     Other embodiments of the invention will be apparent to those skilled in the art from a consideration of this specification or practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.